This is a bit of a trick question – technically, lightning does both. Let’s take a look at the process by which lightning is known to form. This phenomenon occurs due to a difference in charge between a storm cloud and the ground.
Positive lightning travels in reverse, going from the ground to the clouds.
First, the base of a cloud emits a small electrical discharge, called a staggered leader. It descends to the ground in steps, each about 50 yards (about 46 meters) long. This process is extremely fast and impossible to see with the naked eye. Each step is less than a millionth of a second long. The interval between steps is about fifty millionths of a second. This process can only be observed with the aid of extremely fast exposure cameras.
The stepped leader usually moves at about 75 miles per second (120 km/s) towards the ground. The typical duration of a trip is 20 milliseconds. Atoms transmit electrical charge much faster than sound vibrations.
Lightning can travel both up and down.
The staggered leader carries tons of negative charge. As it approaches the ground, it induces huge amounts of positive charge on the ground, especially at the tips of tall objects. As opposites attract, the staggered leader and the negative charge on the ground quickly approach and meet. The path from the storm cloud to the surface is complete and the payload can move.
Storm clouds carry an electrical charge.
Since the cloud is filled with negative charge, it has a lot of current to offer the newly created discharge path. This load quickly goes from being distributed throughout the cloud to being concentrated at the point where the staggered leader first fell from the cloud, to the ground or an elevated object. This discharge is called a knockback and is what we think of when we hear the word “lightning”.
The return blow takes about 100 millionths of a second to hit the ground. The immense flash generated is enough to leave an afterimage in our eyes for seconds at a time, giving us the illusion that the flash is longer than it actually is. In reality, our eyes cannot resolve any of the steps involved. We only see the final product – lightning.